Many industries which utilize microscopic analysis rely upon highly accurate and repeatable measurements of microscopic structures. For example, in semiconductor technology measurements of structures on silicon wafers or mask designs for fabrication of integrated circuits or devices is often necessary. In response to precise measurement needs, various direct measurement techniques utilizing both optical and electron beam scanning devices have been developed. However, due to factors inherent in such systems, these approaches become subject to increasing measurement error as the size of the object being studied decreases. Since projected technological advances, particularly in the semiconductor industry, are expected to lead to even further reduced geometries, alternative techniques are necessary to correct deficiencies found in the known art.
Possibly the most familiar among direct measurement approaches known to the art are optical techniques which employ a laser beam, or other monochromatic ray shaped by a mechanical slit, either of which are scanned across the subject structure in discrete, known steps. The number of sequential steps taken during the scan is simply counted and, for example, when applied in the semiconductor industry, a linewidth can thereby be calculated. Recent improvements to optical systems have provided an increased capability to control measurements. Computers may conform the mechanical slit to the basic shape of the subject structure and then position the slit directly over it resulting in an increase in accuracy. Also, video displays may be used to provide an image on a cathode ray tube (CRT) which is divided into areas of known dimension. Such areas may be counted either manually at the CRT or automatically by computer. Despite these improvements, resolution capabilities ultimately limit all optical systems to geometries no smaller than about 0.4-0.5 micrometers.
To make measurements of linewidths in the half micrometer category and below, scanning electron microscope (SEM) approaches are necessary. SEM systems known to the art operate by scanning a low energy (1-2 keV) electron beam across the structure mounted on a stage. Backscattered electrons and secondary electrons resulting from the physical interaction between the electron beam and the subject structure are sensed by a detector so that signals having a waveform characteristic of the measurement subject may be generated. The signal data is digitized and processed as a video signal to produce an image on a video display screen. A conventional method of SEM linewidth measurement then entails measuring the distance of a reference object whose distance is known and comparing that measurement data with data obtained through performing an identical routine with the subject of interest. Dimensions of the subject are thereby estimates relative to the dimensions of the reference object.
The SEM linewidth measurement method as disclosed in U.S. Pat. No. 4,221,965, also allows correction for tilting of the semiconductor wafer, including the linewidth to be measured, an unknown angle .theta. with respect to the image plane. In that method, two cursors that define the distance of measurement interest are manually aligned with two wafer marks of interest, and a distance measuring circuit or computer then calculates (using simple geometric relationships) the actual distance between the wafer marks (the unknown linewidth) based on those cursor positions, the azimuthal angle .DELTA..theta., and a preset magnification factor.
All of these prior art SEM methods are characterized by their heavy dependency on careful adjustment and monitoring of several parameters influencing magnification. These parameters include the SEM acceleration voltage, focusing coil excitation, and working distance. Typically calibration before each measurement may be necessary due to insufficient monitoring and control of the acceleration voltage or focusing coil excitation. The exact working distance may never be precisely known. Moreover, the prior methods are further subject to error introduced by operator misjudgments regarding focus of either the reference or subject objects and error attributed to misjudgment of distance or alignment on visual displays.
Thus, in order to eliminate the dependence of linewidth measurement techniques on magnification and human factors introducing inaccuracy and nonrepeatability, an improved method and compatible apparatus is disclosed.